flake: tweak nix-filter's list of excludes

flake.nix

@@ -82,12 +82,19 @@
           # In 'nix develop', we don't need a copy of the source tree
           # in the Nix store.
           src = nix-filter.lib.filter {
+            # when in doubt, check out
+            # https://github.com/numtide/nix-filter#design-notes
+            # tl;dr: it'd be best to include folders, however there are
+            # currently issues with that approach.
             root = lib.cleanSource self;
             exclude = [
               ./flake.nix
               ./flake.lock
               ./default.nix
               ./shell.nix
+              ./overlay.nix
+
+              ./README.md
 
               ./.envrc
               ./.drone.star
@@ -98,6 +105,15 @@
               ./.pre-commit-config.yaml
 
               ./.badges
+              # math-optim program output
+              ./out
+              ./res
+
+              # nix result symlink
+              ./result
+
+              # the entire .git folder
+              ./.git
             ];
           };